Effect of plant type and delayed planting on growth and yield parameters of two short day strawberry (Fragaria x ananassa Duch.) cultivars in open field

3.1Plant establishment

Since there was no significant interaction between plant-type and cultivar concerning plant establishment, an average of the two cultivars is presented in Table 2. The first planting date in 2013 was one day after lifting for both bare root plants and plug-plants (Table 1). The plant establishment was close to or higher than 90% for B1, A15, P1, P2 and Pf. Bare root plants stored at 2°C for 14 or 28 days before planting had poorer establishment than the other treatments. Especially ‘Korona’ stored for 28 days was decayed, while most ‘Sonata’ plants survived (data not shown). In 2014, B14 and Pf had poorer establishment than the other plant-types. Storage for 14 days gave poorer establishment than in 2013, probably because the storage temperature was 2°C higher. Again the establishment of ‘Korona’ was much poorer than for ‘Sonata’. Lifting of bare root plants B2 and B3 was undertaken in 2014, but not in 2013, and the establishing was similar to B1. Storage for seven days at 4°C, gave a good result (Table 2). All plug plants established well. However there was poorer establishment for Pf, probably because of warm and dry weather during planting and the following two weeks.

3.2Runner development

Plant type, cultivar and year had influence on runner development, and there was interaction between plant type and cultivar considering number of runners per plant (R/p) and number of propagules per runner (Pg/r) for each establishing year. The main effects of R/p and Pg/r were significant, except for Pg/r for ‘Sonata’ in 2013 (Table 3). A striking effect was the large differences between the two years, with much poorer development in 2013 than in 2014. In 2013, R/p and Pg/r as an average for all plant types, were half the amount or less than in 2014, for both cultivars.

In 2013, R/p for ‘Korona’ was highest for B1, A15 and P1 and lowest for B14, P2 and Pf. Pg/r followed the same pattern to some extent, and were highest in P1,Pf and B1 and lowest in A15 and P2. ‘Sonata’ acted differently and had the highest R/p in B1 and P1, while Pf had the lowest number. Pg/r in ‘Sonata’ was highest in P1 and P2 and B1 and lowest in A15 and B14 and Pf. Plants of B28 died during storage or shortly after planting, resulting in no runners (not tabulated). The average of Pg/r was similar for the two cultivars in 2013, while ‘Korona’ produced more runners than ‘Sonata.

In 2014 the highest number of runners per plant for ‘Sonata’ was on plant type P1, P2, P3, B2 and A13, while B1 and P3 had the highest numbers of propagules per runner; B14 and Pf developed no runners and Pg/r. ‘Korona’ developed in average of plant types, more runners per plant than ‘Sonata’, similar to 2013. Bare-root plants of ‘Korona’ developed the highest number of runners on B1 and B3, but the numbers were highest for P1. The bare-root plant types developed more Pg/r than the plug plants. However, B14 and Pf plants developed no runners or propagules, similar to ‘Sonata’.

3.3Crown-development

There was a trend in 2013, that the ‘Sonata’ B14 plants had crown diameter a little larger than P1 and larger than A15, B1, P2 and Pf (Fig. 1). There were small differences between number of crowns per plant and number of leaves per crown (Table 4). ‘Korona’ had as a trend largest crowns for A15. The plant types B1, P1 and B14 had approximately 3 mm smaller crown diameters than A15, while P2 and Pf had the smallest crowns. However, as a trend B1, P1 and P2 had more crowns than A15, while B14 had only one crown per plant.

Fig.1

Mean crown diameter (mm) and standard error (Se), for plant types of two short day strawberry cultivars in October of two establishing years. B = Bare root; P = Plug; Pf = Fresh plug; A = large bare root (≤13 or ≤15 mm crown diameter in 2014 and 2015, respectively).

In 2014, P2, P1 and B1 of ‘Sonata’ had largest crowns closely followed by B2, B7 and B14 and A13 (Fig. 1). Crowns of P3 and B3 had approximately 1 mm smaller diameter, while Pf had the smallest crowns. There was a trend that plug plants had higher crown numbers than bare root plants except for B7 and B14; Pf had one crown per plant. For ‘Korona’, P3, B7 and P1 had the largest crowns, while A13 and Pf had the smallest. The Pf plants were planted nearly 3 months after B1, A13 and P1, and had 4.7 mm and 1.1 mm smaller diameters than B1 of ‘Sonata’ and Korona’, respectively. There were some differences in number of crowns in ‘Korona’ (Table 4), and B7, B14 and P3 and P1 had the highest numbers, while Pf had only one crown per plant. There were small differences in number of leaves per crown, but Pf had fewest leaves per crown.

Development stage of the primary flower revealed differences between years. The floral development stage was on average 3.3 on 24 October 2013, but 4.1 on the same date in 2014. There was a treatment x cultivar interaction, but no significant difference between cultivars in 2013, but a difference in 2014 (Fig. 2).

Fig.2

Development stage of primary flower in the apex in different plant types of ‘Korona’ and ‘Sonata’ 24 October 2013 and 2014. The standard error bar can be used comparing plant types within and between cultivars. B = Bare root; P = Plug; Pf = Fresh plug; A = large bare root (≤13 or ≤15 mm crown diameter in 2014 and 2015, respectively).

In 2013 B1, P2 and Pf of ‘Korona’ had poorer development than all other treatments. B28 did not develop any crowns in 2013 and the good development of B14 is a result of poor establishment giving better growth conditions for the remaining plants (Table 2). A15 and P1 had had best development for ‘Korona’. For ‘Sonata’ B1, B28, A15, P1 and Pf had the highest flower stage development, while B14 and P2 achieved a lower development stage.

In 2014 B14 of ‘Korona’ did not survive the storage and was not planted, and as a result of that had zero crowns. All other plant types except B7 and Pf reached a development stage close to 4.5, which mean that some of the eight crowns per treatment had development stage 5 (Sepal primordia elongated). For ‘Sonata’, B1, B3, B7 and A13 had the most developed crowns, while B2, P2 and Pf had the poorest development of the plant types. Pf reached only a vegetative growth stage, most likely because it was planted one month later then in 2013.

3.4Fruit yield of bare-root transplants A15 and A13 in the planting year

‘Sonata’ A15 tended to give larger total yield and had larger fruit-weight than ‘Korona’ A15 in 2013, and the percentage of berries larger or equal to 35 cm diameter was higher for ‘Sonata’ than for ‘Korona’ and the opposite for the grade below (Table 5). However, fruit weights were generally low. The percentages of discarded fruit were about equal for the two cultivars, and a little more than 30%. A relative large part of the yield for both cvs was misshapen fruit. One medium strong to small inflorescence was developed per plant (Fig. 3). In 2014, the fruit yield was lower than in 2013. However, the plant type classified as A13 was smaller than in 2013. Similar as to 2013, ‘Sonata’ had higher fruit yield than ‘Korona’, but yields were generally very low. However, there was no difference in fruit weight, although it was also low. Sorting in fruit size classes revealed no differences between cultivars. There were, however, a significant or close to significant interaction for cultivar vs. year for all classes. In average of the two years, ‘Sonata’ had higher yield than ‘Korona’.

Fig.3

Open field flowering in 2013 of ‘Sonata’ bare root A15 plants in Mid–Norway. Photo: Rolf Nestby.

3.5Effects on fruit yield of plant-types and delayed planting the year after planting

Fruit yield the year after planting, was recorded for trial 1 in 2014 and for trial 2 in 2015. There was a striking difference in yield and fruit weight between the years. Trial 1 had lower fruit yield and fruit weight than normal, while trial 2 developed normally. Additionally, there was a significant plant-type vs. year interaction for total yield and fruit weight.

In 2014, there was no difference in total yield between treatments for ‘Sonata’, though B28 tended to have lower yield and larger fruits than the other treatments; ‘Korona’ plant types B1, A15 and Pf yielded more than the other plant types, while the stored plants B14 and B28 had no yield (Table 6). In 2015, ‘Sonata’ B14 yielded less than all other treatments; Pf yielded more than B14, but lower than the other treatments. Plants stored for7 days (B7) had good yield compared to B14 and Bf, and had larger fruits, but lower yield than B1, B2, A13, P1, P2 and P3. In ‘Korona’, B14 had no yield, while B7 with seven days shorter storage, had similar yield as P1, P2 and P3, but could not be significantly separated from B1. For ‘Korona’, as for ‘Sonata’, Pf had lower yield than all other plant types except for B14.

4.1Runner development

The number of runners of all plant-types was about the double in 2014 than in 2013, and similar for the number of propagules per runner (Table 3). The reason for this could be the cold winter 2012/2013 at the transplant production site, which had the following number of days with minimum temperatures below–10.0°C [22]:

The lowest temperature in April was –8.2°C (1 April). In addition, the minimum temperatures in the winter period were rarely above zero degrees before 15 April. These freezing conditions could have caused injury of the transplant crowns and roots and reduced crown growth and development of runners in the summer season of 2013 [23]. Another factor that would increase number of runners in 2014 compared to 2013 was the high July and August temperatures in 2014 (Tab. 7). Conditions that would stimulate formation of runners [14]. Additionally, the soil conditions at the planting site in 2013 were not optimal for establishment and growth of the transplants. The plant type of ‘Sonata’ that developed most runners in 2013 was B1, while Pf developed the fewest runners. The situation was similar for ‘Korona’, except that B14 had very few runners, but the cultivar had more runners than ‘Sonata’. In 2014 the situation was different. Both cultivars developed much more runners than the year before, but ‘Korona’ still more than ‘Sonata’ except for the B2 and A13 plants. ‘Korona’ had more propagules per runner than ‘Sonata’, except for B14 and Pf in 2013. For B14 probably because ‘Korona’ B14 was more weakened during storage than ‘Sonata’ B14. The post-delivery stored transplant B14 did not develop any runners in 2014 for either cultivar, probably because they were weakened during the storage period which was 2°C warmer than in 2013.

4.2Crown development

There were differences between transplant types of ‘Korona’ and ‘Sonata’ in crown development at the end of the first growing season. In 2013, average crown size of plants developed from A15 of both cvs were equal and larger than for plants developed from B1which was planted at the same date. This is reasonable since transplants with large crowns are shown to give a better plant development evaluated by several parameters like leaf area, fruit yield etc. [4–6], and in this case the strong A15 transplants resulted in better plant development than the mixed standard of B1 transplants (6–14 mm). However, B1 of ‘Sonata’ developed better than similar transplants of ‘Korona’. If the grower wants to delay the planting, our results in 2014 show that cold storage of bare root plants (2–4°C) for 7 days (B7) after lifting gave a good result for ‘Korona’ with good establishment after planting and development of a high number of crowns with large diameter at the end of the growing season. In average of treatments ‘Korona’ had smaller crowns than ‘Sonata’ but a higher number, especially in 2014. Some bare root transplants (B2 and B3) were lifted two or three weeks later than B1 in 2014. These plants developed just as well as B1. This indicates that if the grower knows that the planting will be delayed, it is a better option to ask for late delivery instead of storing the transplants at the farm. Late delivery of fresh plug transplants (Pf) like in 2014 when number of crowns was strongly reduced, indicates that fresh plug plants should be planted early in this region.

4.3Flower development in the crown

‘Korona’ and ‘Sonata’ are both considered as facultative short day cultivars, meaning that flower initiation in principle starts when the day is shorter than 15 hrs at temperatures above 18–20°C, while at lower temperatures, some initiate flowers also in long day [24, 25]. The flower-inducing effect of short day is also highly temperature dependent, and at temperatures <12°C and >21°C floral induction is increasingly reduced also under short day conditions [25]. For the cultivars grown in the region of the research station, initiation normally starts in late August [16]. However, just as important as initiation is flower development that depends on temperature in September and October. It is shown that flower bud development increases with higher temperatures [26]. At the latitude of our trials the temperature in September and in October is a minimum factor for development of inflorescences, and was lower in 2013 than in 2014, especially in October (Table 7). Van Delm et al. [27] planted runner tips of ‘Elsanta’ on 23 July for growing tray plants in Belgium. This is at least a month later than in our experiments (except for plant type Pf in 2014) using bare-root plants, and the apical meristem was visible 8 September. On 6 October the apical inflorescence turned into development stage 7. This is much higher than in our experiments, where crowns were examined 24 October, indicating that ‘Elsanta’ is better adapted to a Belgian climate, when it concerns flower initiation and differentiation, than our ‘Sonata’ and ‘Korona’ plants, showing a higher yield potential in late autumn. Therefore, to be able to compete with imported tray plants and waiting bed plants, Norwegian propagators needs to develop a system to improve climate (increase temperature) in autumn [3, 28].

4.4Yield of A15 and A13 plants

The influence of differences in crown size between A plants in 2013 and 2014, is reflected in the yield of A15 and A13 bare root plants (Table 5), with low yields and fruit weights especially for A13 (2014). Even if bare root transplants larger than A15 is produced in Norway with a security of annual delivery, the development of flower buds would be much slower than in Belgium, especially with obligate short day cvs like Korona and Elsanta. Even at the south part of Norway crowns with diameter larger than 15 mm is hard to produce without using artificial means (production in tunnels etc. to increase temperature). The field trials showed that we achieved a total yield in 2013 of more than a kilogram per m^–2 (0.227 kg plant^–1) of ‘Sonata’, but only 40% of that for ‘Korona’, and the fruit weight was low. In 2014, the crown diameter was 13 mm and the difference between the cvs was much the same, but yields were disappointingly low; in spite that the average temperature for September and October in 2013 was 10.8°C compared to 9.2°C in 2012 [22], conditions that should favour (but still too cold for optimal development) better flower bud development in late autumn 2013 and higher yield in 2014. The low yield especially in 2014 was partly because the average fruit weight was low, but also because the plants produced only one small sized inflorescence per plant with 5–12 flowers (Fig. 3). Nurseries further south in Europe delivered A+ (11–13 mm) plants with approximate 18 flowers per plant and A++ (16–18 mm) with43 flowers per plant [2]. At Norwegian nurseries they normally remove the first runners because they are few, and therefore produce transplants on later developing runners to secure a large crop of transplants that they sell as a mixed plant size. If they start to produce transplants from the first runners and move plants into polytunnels, it could be possible to deliver A+ plants or preferably A++ plants at a quality of interest to Norwegian growers. However, they would have to compete with a high quality plant from nurseries further south in Europe, delivering a defined transplant quality.

4.5Yield influenced by plant-type treatment

In the traditional concept of growing short day strawberry plants, as in our experiments, with an establishing year and a cropping year, fruit yield parameters were influenced by year, plant-type and cultivar and there was interactions year vs. plant-type concerning total and marketable yield (Table 6). In trial 1 (2014), total and marketable yield were much lower than normal. Plant types of ‘Sonata’ did not react much different from ‘Korona’, except that the cold stored plant types B14 and B28 of ‘Sonata’ gave some yield in contrast to ‘Korona’ that did not yield any fruits, and had roughly 66.8% and 60%, respectively, of the yield compared with the other treatments. This is a result of poor plant establishment in 2013 especially of ‘Korona’ (Tables 2 and 6). And there was not a significant effect on fruit weight by treatment for both cvs. However, in ‘Sonata’, B28 tended to have the largest fruits, and the reason for this is probably that each plant had less competition than in a fully established plot; and Pf tended to have the smallest fruits probably because of a smaller crown diameter (Fig. 1). ‘Korona’ reacted differently and produced higher yields for B1, A15 and Pf than for the other plant-types, and higher yield than ‘Sonata’. The plug plants of ‘Korona’ yielded similar to ‘Sonata,’ except for Pf which had the highest yield in trial 1 for ‘Korona’ in spite of small crown diameter in the autumn of establishment (Fig. 1). The reason of the relative high yield of Pf is probably that Pf-transplants were healthy and without winter injury,the planting date 26 June was also early enough to secure a relatively high yield potential [23]. The relatively low yield of P1 and P2 plants is probably caused by freezing of the plants in spite several layers of fleece covering the plugs at the nursery during winter. The roots and the crown of the bare root transplants are better protected than for plug plants since they grow in soil and are covered by fleece. Freezing injury at the nursery in addition to poor planting conditions in 2013, are probably the reason of the low fruit weights.

In trial 2 (2015), the conditions for growth were normal, with the highest yields beyond two kg m^–2 for both cultivars. For ‘Sonata’, plant type P3 gave highest yield, but with the data available it is not possible to fully understand the reason why it produced more than P1 and P2 in spite of later planting date and smaller crowns, but the number of fruits per m^–2 was higher. The relatively low yield of plant-type Pf, however, must be related to the late planting and the small crown diameter (Table 1, Fig. 1), which is expected to give lower yield than earlier planting [23]. The three bare root transplant treatments B1, B3 and A13 had equal yields, but much higher than for B1 transplants stored for 14 days (B14). The reason is weakening of the plants and rotting at storage, and therefore poor establishment (Table 2). The bare root transplants stored for 7 days (B7) at 4°C established well, but yielded lower for ‘Sonata’ than all other bare root transplants except B14. It is obvious that storage even for seven days under the actual conditions is not recommendable for ‘Sonata’. ‘Korona’ bare root transplants produced yields comparable to ‘Sonata’. However, B14 produced no yield while B7 produced yields equal to or higher than mixed bare root transplants and A13. This suggests that ‘Korona’ could be stored for seven days at the given conditions without yield reduction. The Pf-transplants gave low yield, though not as low as for ‘Sonata’, but lower than for normally treated bare root and plug transplants because of late planting date. There were differences in fruit weight between ‘Korona’ plant types, and Pf, B2, A13 and P2 had the smallest fruits, while B7 and P1 had the largest fruits. The effect on Pf is explainable because of small crown diameter and P2 had many crowns competing which could explain reduction in fruit size. For ‘Sonata’, there was (because of a much larger variation within treatment than for ‘Korona’) low fruit weight of Pf plants and high fruit weight of B14. The effect on Pf plants is probably connected to relatively poor crown development shown by small crown diameter, few crowns per plant and few leaves per crown compared with the three other plug plant-types. (Tables 4 and 6, Fig. 1).